This invention relates to optical viewing devices. More particularly, it relates to sighting and position determining devices such as optical compasses.
In the use of scientific instruments having meters, quite often inaccuracies in reading the meters caused by parallax are greater than the internal inaccuracies of the device itself. Parallax occurs in reading instruments such as compasses because the needle does not lie directly against its scale. This parallax phenomenon can be readily observed in reading the speedometer of an automobile whereby the driver of the automobile observes one speed while the passenger perceives another less accurate reading. Unless the observer's eye is aligned correctly with the needle and the scale, parallax will cause inaccuracies in reading the instrument.
Another problem primarily associated with compasses is that the compass magnetic needle is normally operative only in the horizontal plane; therefore, the needle is not directly accessible for sighting an object. Some improvements in sighting accuracy have been made, such as by utilizing a fold-out mirror which is integral with the compass housing which allows a person to sight an object adjacent to the mirror image of the compass reading. However, that type compass does not solve the problem of parallax.
In the field of navigation, compass reading inaccuracies caused by phenomenon such as parallax can be devasting. For example, a one-degree error results in a 88-foot inaccuracy for every mile. Thus in a 100-mile voyage the inaccuracy is approximately 11/2 miles. This type of inaccuracy is graphically demonstrated when a prior art compass is used in sighting distant objects such as mountain peaks for determining the position of the user by triangulation. Furthermore, the larger the distance between a compass needle and the compass face, the greater the parallax problem.
U.S. Pat. No. 4,402,140 issued to Nagae shows a telescope which utilizes an immersion type spherical compass so that a compass reading may be made even if the device is not in the horizontal position. However, the Nagae patent does not overcome the parallax problem.
U.S. Pat. No. 3,084,443 issued to Kaatz teaches a compass which also utilizes a floating sphere and which also includes latitude lines so that the device may also be used as a inclinometer. However, there is no provision for dealing with the parallax problem.
British Pat. Nos. 1,293,808 and 1,510,467 and U.S. Pat. Nos. 1,043,562, 2,022,516 and 2,970,510 also show compasses which utilize optical systems. However, again, none of these patents solve the parallax problem.
U.S. Pat. Nos. 1,376,727, 3,128,562 and 1,294,710 show compasses which utilize floating spheres so that readings may be made even though the compass is not oriented horizontally.
In the field unrelated to compasses, U.S. Pat. No. 2,911,879 teaches the use of a beam splitter to superimpose the image of a grid on to the image of an object being viewed. However, parallax is still a problem in this patent.